1. Field of the Invention
The present invention relates to an electrostatic discharge (ESD) protection device and a method for manufacturing the ESD protection device, and particularly to an ESD protection device having improved ESD characteristics, such as discharge starting voltage, and reliability in which discharge electrodes are disposed in a cavity of an insulating substrate so as to face each other.
2. Description of the Related Art
ESD is a phenomenon in which strong discharge is generated when a charged conductive body (e.g., human body) comes into contact with or comes sufficiently close to another conductive body (e.g., electronic device). ESD causes damage or malfunctioning of electronic devices. To prevent this, an excessively high voltage generated during discharge must be prevented from being applied to circuits of the electronic devices. ESD protection devices, which are also called surge absorbers, are used for such an application.
An ESD protection device is disposed, for instance, between a signal line and a ground of a circuit. The ESD protection device includes a pair of discharge electrodes facing each other with a space provided therebetween. Therefore, the ESD protection device has high resistance under normal operation and a signal is not sent to the ground. An excessively high voltage, for example, generated by static electricity through an antenna of a cellular phone causes discharge between the discharge electrodes of the ESD protection device, which leads the static electricity to the ground. Thus, a voltage generated by static electricity is not applied to the circuits disposed downstream from the ESD protection device, which protects the circuits.
For example, an ESD protection device shown in an exploded perspective view of FIG. 37 and a sectional view of FIG. 38 includes a cavity 5 provided in a ceramic multilayer substrate 7 including a plurality of laminated insulating ceramic sheets 2. Discharge electrodes 6 facing each other and electrically connected to external electrodes 1 are disposed in the cavity 5 that includes a discharge gas. When a breakdown voltage is applied between the discharge electrodes 6, discharge is generated between the discharge electrodes 6 in the cavity 5, which leads an excessive voltage to the ground. Consequently, the circuits disposed downstream from the ESD protection device are protected (see, for example, Japanese Unexamined Patent Application Publication No. 2001-43954).
However, in such an ESD protection device, the discharge responsivity to ESD varies significantly due to the variation in the distance between the discharge electrodes. Furthermore, although the responsivity to ESD needs to be adjusted using an area of the region sandwiched between discharge electrodes facing each other, the amount of adjustment is limited by the size of the product or other factors. Therefore, it may be difficult to achieve a desired responsivity to ESD.
Furthermore, a discharge phenomenon is efficiently caused by employing a structure in which a conductive material is dispersed between discharge electrodes. However, with such a structure, since the conductive material is scattered due to an impact caused during discharge and thus the distribution density is decreased, the discharge voltage is gradually increased each time discharge is performed. Consequently, discharge characteristics are degraded after repetitive discharges.
Moreover, such an ESD protection device poses the following problems.
Firstly, a discharge starting voltage is primarily set by adjusting the distance between discharge electrodes. However, the distance between discharge electrodes is varied because of variations that occur during production and a difference in shrinkage behavior between a ceramic multilayer substrate and discharge electrodes during firing. Consequently, the discharge starting voltage of an ESD protection device varies. Therefore, the discharge starting voltage cannot be set with high precision.
Secondly, discharge electrodes in a cavity may be detached from a ceramic multilayer substrate because of a decrease in the hermeticity of the cavity or a difference in a coefficient of thermal expansion (may be referred to as “thermal expansivity”) between a base layer of the ceramic multilayer substrate and the discharge electrodes. In such a case, the device becomes unusable as an ESD protection device, or the discharge starting voltage is changed and, thus, the reliability of the ESD protection device is degraded.